The present invention relates to the application of machine vision algorithms in the manufacture of semiconductor devices. Specifically, the invention applies to the inspection of interconnection lands, or pads, of integrated circuits during fabrication, to ascertain that mechanical probe marks exist, indicating that an electrical test has been performed. The use of an inspection apparatus that can automatically detect and analyze probe marks on device lands can be more reliable and accurate than manual inspection with the human eye.
Silicon device fabrication involves various circuit definition steps with chemical processing of a wafer containing an array of multiple devices. An electrical test of the wafer is typically employed to identify defective devices and/or to perform process control. This electrical test requires that an unbroken electrical connection be made from an electrical tester to the device being tested. The electrical connection is accomplished by probing the wafer with needle-tipped, electrically conductive probes, leaving an indentation or surface finish aberration on the pad where the probe made contact with the pad. An inspection of the mark left by the electrical testing probe may be performed subsequent to the electrical test to visually assess the electrical testing operations. Absence of a mark, or misalignment of the mark on the pad, would indicate that the test was not properly performed. Probe marks that are too large, or near the pad border can damage the integrity of the pad. Additionally, data collected from a probe mark inspection may be useful in process characterization and process control of equipment alignment parameters.
Probe mark inspection is a useful operation in silicon device fabrication because of the increasing size, density, and value of the silicon wafers. Good devices must not be falsely identified as defective, whereas defective devices must be accurately identified so that needless processing and packaging effort is not further expended. Verification of the integrity of the electrical testing operation ensures that the devices are properly tested to avoid the cost penalties of false or improper indictment. Human visual inspection using a microscope is extremely tedious. Thus, the operation is ideally suited for a computer-assisted machine vision application.
Inspection of probe marks on device pads with a machine vision system is difficult because probe marks are relatively unstructured in appearance. A mark indicating acceptable evidence of a probe mark may present many different appearances in an image. Additionally, the pad image may contain features from surface imperfections of the pad that may confuse or obscure the interpretation or presence of a probe mark. Furthermore, particulate deposition on the device pads from the environment may appear similar to the image of a probe mark.
Further still, the surface of the interconnection pad may be conditioned to exhibit texture or features that further complicates the inspection of probe marks. A textured surface, desired to improve the interconnection bond strength of assembled package, appears to have regular structure features in a typical grayscale image of the pad, as depicted in FIG. 2. The application of prior art methods of probe mark inspection using machine vision will often yield erroneous results since the texture features are misinterpreted to be probe mark features.
For the foregoing reasons, there is a need for a machine vision apparatus that can inspect textured silicon device pads on a wafer, and quickly and reliably assess the presence of a probe mark.